Certain wearable computers such as those embodied as eyeglasses or virtual technology goggles project an image directly in front of a user's eye. In eyeglass type devices these projections are see-through so the user can see the projected data in the near field while the visual real-world in the far field remains largely unobscured. In virtual reality devices the user is isolated from perceiving the real world so the display needs to fill the user's entire field of vision. One challenge with such wearable displays is to produce an adequate eye-box in which the viewer can view the data that is projected by the micro-display. Such an the eye-box for see-through displays measures about 10-12 mm in the vertical and in the horizontal and the eye relief is in the range of 20-30 mm. For virtual reality devices the eye box is necessarily larger and often the eye relief is a bit longer. Retinal scanning display devices project the image directly on the user's retina so the eye-box is smaller and the eye relief is closer to zero. Due to the nature of such wearable devices the space constraints limit the reach of the optics and so one challenge is to keep that eye-box from shrinking to only a few mm, given the optical train (often located at the side of the user's head for see-through displays) is limited by practical limits to the size of such wearable devices. These size limits to the optical train also adversely affect the color space seen by the user. Color space may be a peripheral matter for see through displays where only data is being displayed but is critical for virtual reality devices whose effectiveness relies on the display persuading a certain level of the user's consciousness that the scene represents more than only a virtual world.
The exit pupil expander (EPE) is the optical component that would replace the geometric optics that have traditionally been used to expand the size of the eye-box in head-wearable visual devices. In optics the exit pupil is a virtual aperture in that only rays which pass through this virtual aperture can exit the system. The exit pupil is the image of the aperture stop in the optics that follow it. The term exit pupil is sometimes also used to refer to the diameter of the virtual aperture. Unlike the optics of conventional cameras or telescopes, an exit pupil expander of a wearable virtual reality or see-through device is designed to display for near-distance viewing.
Numerical aperture expander is a less common term sometimes used with reference to retinal scanning displays which project an image through the pupil directly on the user's retina. The numerical aperture of the light emanating from display pixels determines the exit pupil size, and retinal scanning displays project a rastered image about the size of the user's eye pupil at an intermediate plane. Retinal scanning displays can be used for virtual reality applications.
Diffractive exit pupil expanders have diffraction gratings that pose an inherent problem in controlling the color space. Because of diffraction the input and output gratings diffract different color bands of light into different output angles. This results in the user's perception of the color space of the scene being displayed having a varying color balance across the user's field of view.
Conventional exit pupil expanders typically have a very high degree of parallelism which FIG. 1 demonstrates with parallel front and back surfaces of the EPE. Incident light 102 enters the EPE 100 via the back surface 104 and encounters an input grating 106. Light propagates inside the EPE 100 by multiple total internal reflections (TIR) and the color space is controlled by having a stack of EPE plates, for example separate plates for red (R) and green (G) as well as blue (B) primary color bands. Light exiting the EPE 100 is expanded by these internal reflections and passes through an output grating 108 and exits normal to the front surface 110, which is parallel to the opposed back surface 104. This plate stacking necessarily complicates the design and raises its cost. The individual beams in FIG. 1 represent different colors (R, G, B) each defining a different wavelength λ.